They are naturally related but I see important daylight between them. I see no problems they're causing. (And if there's a problem to solve, and we were to combine them, the synonyming should retain the more general attendance and eliminate the narrower absent-players.)
Detailed in reverse order:
If forced to pick one, the concept “attendance” is more useful than “absent players” for tagging
Absent players is a subset of attendance issues, and if we were to keep only one, we should keep the tag that can already cover both and make the other a synonym. That would mean making attendance the main tag and making the synonym absent-players → attenance.
There's no problem they're causing
The only problem I can see them causing is redundancy, assuming for the moment and the sake of argument that they're not distinct. If they are distinct, being small isn't enough to get rid of a tag, if it accurately describers a common subject of a collection of questions.
Without problems such as causing harm or being redundant with existing tags, there's no reason to get rid of a tag.
They're distinct enough
The tag attendance covers more than just the problem of absent players: it covers figuring out attendance, how to encourage attendance, how to schedule groups to ensure attendance (rather than other problems related to scheduling), how to manage games where attendance is designed to be variable (which doesn't qualify as players being “absent” — absence implies you're supposed to be present).
Meanwhile the tag absent-players describes a potential subset of attendance, but it's a distinct and acute problem that RPG players encounter regularly. To my eyes that immediately promotes the subject of absent players to being an important issue all by itself that can have experts who have solved the problem before, distinct questions that should be interconnected in our system, and a specific first-class problem category that should be easily found directly rather than as part of a more general “attendance” question collection.
Absent players is a high-profile problem that GMs and groups do have, and it deserves the high profile of having a tag here. Attendance is also an ongoing source of problems and difficulties that aren't always about absence, and so it is distinct too and can't be readily dispensed with.
Therefore: We should keep them both
They're both useful, and neither are causing problems. If they're not being applied consistently, we aware tag-managers can edit to make them more consistent — one of the very first editing privileges we give to users.